The present invention relates to an apparatus and method for learning gear ratio control of a CVT (Continuously Variable Transmission), and more particularly, to an apparatus and method for learning CVT gear ratio control, which can prevent a problem through target gear ratio control learning, the problem occurring when a turbine shift curve is delayed or advanced during pulley gear shifting in which a gear ratio is significantly changed due to the driving state of a CVT vehicle or the wear and deterioration of a contact surface between a pulley and a belt.
In general, an automatic transmission vehicle is driven under control of an electronic controller referred to as a TCU (Transmission Control Unit), and the TCU controls overall operations of the automatic transmission. At this time, the TCU electronically controls the automatic transmission to shift gears according to the velocity of the vehicle and the throttle opening degree. When the vehicle is driven by the power of the engine during the gear shifting, the state is referred to as a power-on state. When the vehicle is not driven by the power of the engine, but driven by the inertia of the vehicle (coasting), the state is referred to as a power-off state. Since an engine RPM, engine torque and engine power are different during the power-on state and the power-off state, gear shifting controls suitable for the power-on state and the power-off state need to performed.
In general, when a driver takes his foot off the accelerator pedal during driving (lift foot up), the operation state of the vehicle is switched from the state in which the engine drives the vehicle into the power-off state in which the vehicle drives the engine, and the speed stage is changed to a high-speed stage higher by one or more stages than the current speed stage. At this time, the number of turbine revolutions decreases, and gear shifting control accompanied in this case is typically referred to as power-off up-shift.
For example, a typical automatic transmission is automatically synchronized with a target speed stage through a gear shifting pattern of a map table, which is set according to changes of the vehicle velocity and the throttle opening degree. The automatic transmission applies the power-off up-shift control method which changes the speed stage to a speed stage higher by one or more stages than the current speed stage, in order to exclude an occurrence of shock by an instantaneous engine power reduction in case of the power-off state in which the driver takes the foot off the accelerator pedal.
In the existing CVT, however, a gear ratio control learning process for specific pulley gear shifting is not performed, but only a pulley control pressure learning process of comparing control pressures and pressure sensor values of a driving shaft and driven shaft in the parking position P or neutral position N is performed as a learning process used for the pulley. Since the gear ratio control learning process for pulley gear shifting is not performed, gear shifting correction into which a driver's intention is reflected depending on the wear of the transmission or the surrounding environment is not performed. Therefore, the gear shifting of the transmission may delayed or rapidly performed to cause a shock during the gear shifting control.
Therefore, the gear ratio control learning is required to perform transmission control into which a driver's intention (for example, an intention to coast the vehicle) is more reliably reflected, through the target gear ratio control learning process (i.e. learned target gear ratio).
The related art of the present invention is disclosed in Korean Patent Publication No. 10-2006-0108867 published on Oct. 18, 2006 and entitled “Power-off up-shift control method of automatic transmission vehicle”.